The present invention relates to an electrophotographic recording material including a dual photoconductive layer applied to and electrically conductive substrate, with each one of the two photoconductive layers in the dual layer containing selenium. The dual layer contains a lower photoconductive layer disposed on the substrate and made of an amorphous system comprising arsenic and selenium and an upper photoconductive layer which is made of As.sub.2-x Bi.sub.x Se.sub.3 or of As.sub.2 Se.sub.3-y Te.sub.y.
Electrophotographic recording materials are used for electrophotographic copying processes which have found wide application in the photocopying art. Such processes are based on the property of the photoconductive material to change its electrical resistance when exposed to an activating radiation.
After a photoconductive layer has been electrically charged and exposed to an activating radiation in a pattern corresponding to an optical image, a latent electrical charge image, which corresponds to the optical image, is produced on the photoconductive layer. At the exposed locations, the conductivity of the photoconductive layer is increased to such an extent that the electric charge can flow off, at least in part, through the conductive substrate, but in any event the flow-off is at a greater extent at the exposed locations than at the unexposed locations. At the unexposed locations, the electric charge should remain essentially intact, and the pattern of the charge can then be made visible by means of an image powder, a so-called toner. The resulting toner image, if necessary, can then be transferred to paper or a similar record carrier. Electrophotographically active substances which have been employed include organic as well as inorganic substances. Among the inorganic substances which have been used, selenium, selenium alloys and compounds with selenium have gained particular significance. They play an important role particularly in their amorphous state and have found many uses in practice.
The change in the electrical conductivity of a photoconductor depends on the intensity and the wavelength of the radiation employed. Within the range of visible light which is preferred for practical use in electrophotography, for example, in office copiers, the amorphous selenium exhibits high sensitivity on the blue side, i.e. in the shortwave range, whereas on the red side, i.e. in the longwave range, it exhibits very low sensitivity.
The result is that a red character is reproduced on an electrophotographic plate in the same manner as a black character, which under certain circumstances, particularly with colored masters, may present practical disadvantage, since a black character on a red background--or vice versa --will not be distinguishable from its background and cannot, therefore, be made visible. For wavelengths in the infrared range, amorphous selenium is not suitable at all.
However, those are the sensitivity ranges which are desirable if electrophotography is also to be used to advantage for other purposes, for example for recordings made with laser radiation.
It is known that data output devices use IR solid state lasers which operate as radiation sources. Compared to gas lasers, IR solid state lasers have the advantage of direct modulation of the light emission via the diode current and they are additionally distinguished by a relatively low price. For that reason it is desirable, even preferred, to use solid state lasers. Since, however, solid state lasers emit only in a spectral range starting at about 760 nm, i.e. in a spectral range in which most of the photoconductors customarily used in electrophotography have only slight absorption and, correspondingly, only slight photosensitivity, and since the reduced photosensitivity of the photoconductors can be compensated only in part by the higher intensity of the laser radiation, electrophotography can be used expediently and advantageously for recordings made with laser radiation only if the sensitivity of the photoconductors can be extended into the IR range.
It is known that, in contrdistinction to amorphous selenium, crystallized selenium is red sensitive. Therefore if crystallized selenium is used, this part of the visible spectrum can also be utilized. However, the high dark conductivity of crystallized selenium, i.e. its characteristic of conducting electrical current very well already in the unexposed state so that a charge applied to its surface cannot be held for the length of time required for electrophotographic purposes, speaks against the use of crystallized selenium for such electrophotographic purposes.
It is also known, for example from DE-AS 2,248,054 and DE-AS 1,597,882 that spectral sensitivity can be extended into the longer wave spectral range by adding substances such as arsenic and/or tellurium to the selenium. A system made of selenium and tellurium, however, has the drawback that it is more difficult to homogeneously evaporate these materials in the form of an alloy. Moreover, with higher tellurium concentrations, the photoconductive layer exhibits an undesirable tendency to crystallize and thus has only a short service life.
DE-OS 30 20 938, DE-OS 30 20 939, and U.S. application Ser. No. 06/269,941, filed on June 3rd, 1981 and corresponding to DE OS 30 20 939, disclose dual layer structures in which the lower layer is made of amorphous arsenic selenide. The upper layer on top of this arsenic selenide layer is made of a compound of arsenic, bismuth and selenium of the general formula As.sub.2-x Bi.sub.x Se.sub.3 with x values between 0.01 .ltoreq..times..ltoreq.0.5, or of a compound of arsenic, selenium and tellurium of the general formula As.sub.2 Se.sub.3-y Te.sub.y with y values between 0.05.ltoreq.y.ltoreq.2.5. These dual photoconductor layers comprising a lower charge carrier transporting layer and an upper charge carrier generating layer exhibit a noticeable and practically utilizable sensitivity extending into a wavelength range of more than 800 nm.
The realization of a particularly high photosensitivity which extends into the IR range, however, brings with it a certain fatigue of the photoconductor material, i.e. an increase in dark discharges and a connected reduction of the charging potential under cyclical stress.
It can be assumed that the occurrence of fatigue under cyclical stress is a direct result of the extremely poor mobility of the electrons in the photoconductive material. As a consequence of this poor mobility, negative space charge zones build up in the interior of the photoconductor near its exposed surface and these space charge zones cause increased injection of positive surface changes so that the above-mentioned reduction in charging potential becomes evident as a loss of contrast in the printed image. This effect is somewhat augmented if, for the purpose of expanding the sensitivity range, the photoconductor contains tellurium or bismuth or antimony.
It is possible, by means of suitable measures, to sufficiently stabilize the charging potential even under continued cyclical stress, for example by selecting an erase light system whose spectral emission is set in such a way that it stabilizes the density of the space charge, and/or by the use of charging devices, such as the Scorotron, which keeps the surface potential constant.
This stabilization, however, applies only as long as light is used which comes from a radiation source that is greatly absorbed by the photoconductors so that photoelectric electrons are generated only in an extremely thin zone closely below the exposed surface of the photoconductor. These electrons produce the photodischarge of the positive surface charge and thus do not contribute to a change or buildup of space charges in the interior of the photoconductor.
If, however, light is used for which the photoconductors have less absorption capability, electron-hole pairs are formed not only immediately below the surface but also in the interior of the photoconductor. Corresponding to their positive charge, the holes travel to the substrate while the electrons, due to their very poor mobility, build up space charges at the illuminated locations in the interior of the photoconductor, with these space charges depending on the intensity and areal expanse of the illuminated regions. As a consequence, additional fatigue occurs in dependence on intensity and location.